Multiple feedback system



July 28, 1953 MULTIPLE FEEDBACK SYSTEM 2 Sheets-Sheet 1 Filed Oct. 28, 1948 E c FH s AGE, ND

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I M: 200 N I00 lvzvnt wwz CMQMQS Ali @uWtiw@w-@t my zzmzzl a) Patented July 28, 1953 2,647,173 MULTIPLE FEEDBACK SYSTEM Charles A. E. Beurtheret, Paris, France, assignor to General Electric New York Company, a corporation of Application October 28, 1948, Serial No. 57,001 In France November 17, 1947 v 1 p s My invention relates to amplifying systems and, more particularly, to systems using inverse voltage feedback.

Conventional broadcast transmitters usually use high level plate modulation in the final stage. Inverse voltage feedback may be used to obtain improved performance. However, in the past the amount of feedback voltage used has been limited due to the tendency of the amplifier to oscillate if the feedback voltage rises over a certain value.

It is an object of my invention to provide an improved amplifying system utilizing negative feedback, particularly useful in a transmitter.

A further object of my invention is to provide an improved multiple feedback system.

A further object of my invention is to provide means for substantially reducing signal distortion in an amplifier.

For additional objects and advantages, and for a better understanding of the invention, attention is now directed to the following description and accompanying drawings, and also to the appended claims in which the features of the invention believed to be novel are particularly pointed out.

Fig. 1 is a schematic diagram illustrating one embodiment of my invention;

Fig. 2 is a circle voltage diagram;

Fig. 3 is a curve showing the amplification characteristics of the auxiliary amplifier of Fig. 1;

Fig. 4 is a schematic diagram of the addition circuit of Fig. l; and

Figs. 5, 6 and '7 are Nyquist diagrams to explain the operation of my invention.

In Fig. 1 'I have illustrated, schematically, a transmitter comprising an amplifier stage I, audio power amplifier stage 2, a modulation transformer 4, a modulation reactor 5, a coupling network 6, radio frequency stages I and an antenna 8. The radio frequency stage I may conventionally include an exciter, modulated stage and radio frequency power output stage.

A suitable source of high voltage supply is designated by the conventional symbol B+. The simplified blocks are for conventional elements and a detailed description will be omitted for simplicity. The coupling network 6 comprises inductance 9, capacitor 10 and resistance II. A feedback voltage is obtained from the resistor II and is supplied through a phase-adjusting circuit l2 to an addition stage 13. Additional signal feedback is obtained from the output stage 1. The output signal is demodulated in detector 4 Claims- (Cl. 179-171) andsupplied to the stage l3 through a phaseadjusting circuit I6. An auxiliary amplifier I1 is connected, in parallel, withthe amplifier 2'. The output of the amplifier l! is supplied to the stage l3. The output of I3 is' returned as a'feedback signal and combined with the incoming audio signal.

The amplifier-l receives the incoming audio signal from a transformer l8. The output of the amplifier is connected to the grids l9 and of the amplifier devices 2| and 22 which are connected in a conventional push-pull manner.

The transformer 4 transfers at high level the audio modulation to the final radio frequency stages 1 of the transmitter through a network consisting of the modulation transformer 4, a reactor 5 and a blocking condenser 23 by means of a coupling network 6 and thence to the antenna 8. The inductance 5 constitutes the mod 20 ulator choke in the anode circuit'of the modulator stage. The final stages I comprise the major portion of the load on the'modulator stage. Ordinarily, the impedance presented by the final stage varies with frequency in such a manner that over the audio band it is primarily resistive.

The coupling circuit 6 possesses the general characteristics of a low pass filter and consists, essentially, of inductance 9 in series with the resistive load presented by the final stage 1 and a shunt circuit consisting of capacitor 10 and resistance 'H in series circuit relation. The circuit 6 is essentially of the type commonly known as a monocyclic network and presents a purely resistive impedance between point D and ground. This is accomplished by proportioning the network constants so that the resistance presented by the final stages I and theresistance II are equal and, in turn, are equal to the square root of L/C where L and C are the inductance and capacity of the elements 9 and Hi. In order to limit the power dissipated in the resistor H, the elements 9 and [0 are so chosen that their resonant frequency will lie well above the audio band. This circuit 6 thus possesses the fundamental characteristic that it presents a constant resistive loadimpedance to the modulator stage independent of frequency. In addition, theshunt arm comprising the capacitor lll and resistor H forms a circuit in which high frequency distortion components are prevented from entering the final radio frequency stage as they form a low impedance path to ground.

Feedback voltage is also derived directly from the final output/power amplifier by means of detector I5. The voltage produced is proporover the feedback frequency spectrum. In the region of the audio band; the::vector"sum. g is supplied almost exclusively'by theafeedback from the final stage there being a slight phase correction in the phase-adjusting circuit. 16.

At the high frequencies above the audioband, g is composed substantially of the frequency components derived from the resistance H: as the voltage across I I increases with frequency. A

proper voltage levelis maintained by tapping the resistance H at asuitable point 24 so that the circle diagram of Fig.- 2 'is'obtained.

Stray capacities between the point c and ground have very little effect upon the system and do not materially affect the principle-of operation. Usually, the phase adjusting-circuits l2 and I6 allow sufficient compensation for thisstray capacity.

The feedback frequency spectrum contains frequencies that are'both above and below the audio band. The problem' of stability is concerned mainly with those frequencies which lie outside of the. audio band. Bythe proper control of those frequencies outside of the band, the amplifier. will bestable whenfeedback is applied. If thefeedback were.-taken from the point and applied directly. tor. the point It, the amplifier wouldproba'blyoscillate as is-illustrated in the Nyquist diagram of- Fig. 5. It'can be seen that the Nyquist point 1+7'O is included within the diagram, thus'satisfying:therconditionfor oscillation.

The circuit l3 adds vectorially the-voltages e and: 1 derived from the output'circuit to a third voltage h from an auxiliary amplifier H. The input of this auxiliary amplifier is connected to the same point as'that of the" amplifier l. The amplifier l1 possesses atransmission-characteris-- tic as illustrated in Figs. 6 and 3. This characteristic is so chosen that the vectorial addition of- Figs. 5 and 6 will result in a Nyquist diagram as illustrated by Fig. 7. The Figs. 5, 6 and llare drawn to cover thepart. of the frequency spectrum above: the=audio band. Similar, figures in the appropriatequadrants may be. obtained by similar methods for the. frequency region lying below the audio band. In the audio band the amplifier I! has substantially zero amplification but possesses substantial amplification at high and low frequencies, as illustrated by the curve of Fig. 3, so that the vector addition of the voltages'h, f and e will result in a Nyquist diagram in which the point 1+7'0 is always outside of the diagram. Thus, it can be seen that it is possible to increase the amount of inverse feedback in the audio band by a substantial amount and thereby improve. the characteristics of the amplifier.

Itis not necessary that the frequencies f1 and f2, asshown in Fig. 3, correspond exactly to the frequencies at which the amplifier would oscillate were the amplifier. l'! and the feedback circuits not present. For practical. operations these. frequencies need be only approximate.

In Fig. 4, I have shown a schematic diagram of 1c The stage I3 adds vectorially thevoltagesaand.

the addition circuit l3 which comprises three pentode discharge devices in which the voltages e, f and h are impressed on the grids 25, 26 and 21 of three respective devices 28, 29 and 30 whose anodes 3|, 32 and 33 are connected to a common anode resistance 34. It can be seen that with this arrangement the voltageappearing across the resistor=34 will be the .vector. sum of F e; f and h.

The application of this system permits a substantial increase in the inverse voltage feedback making possible substantial economies in manufacturing iii-that the filtering of the high voltage supply may be reduced and also it permits a large amount of; filament? hum in the radio frequency amplifier devices;

Modulation transformers and reactors in which distor.tion,..due to poor magnetic material, would be excessive when used with conventional amplifier'systems, can be used with my system without reduction in performance.

While certain-specific embodiments havewbeen shown.- and described, it will, of. course, be. understood that various modifications may be. made Without departing. from the invention. The appended claims are, therefore, intended to cover any such modifications within the true'spirit and scope of the invention.-

What I claim-as new and" desire to secureby Letters Patent of the United States-is:

1. In. an amplifier system, a source of signal voltages, a first amplifier, a second amplifier; means for impressing said signal voltages across the input-. of said amplifiers, a radio frequency stage, a constantimpedance coupling circuit connecting the. output ofsaid first amplifier to said radio frequency stage, saidcoupling circuit comprising an inductancein series with saidoutput and. a capacitance. and resistance. forming a shunt branch, means for deriving a feedback voltage from said shuntbranch, means for deriving. a secondfeedback voltage from saidradio frequency stage, means for combining said-feedback voltages with the output voltage of said second amplifier, means for impressing the. resultant voltage upon the input of saidamplifiers to provide substantial feedback voltage thereby-to improve the stability of saidamplifier.

2. An amplifier system for amplifying voltages extending over a band of frequencies, a main amplifier having aninput and an'output circuit, a load-circuit, a coupling. network connecting. said output circuit to said load circuit, means for deriving-from said load circuita-voltage decreasing with increase in frequency beyond said band, means for deriving from said couplingnetwork a second voltage. increasing with-increasev infrequency beyond. said band, meansfor adding said voltageswhereby the phase of the resultant voltage does not vary. appreciably over a range of frequenciesbeyond. said band, an auxiliary amplifier. having an inputcircuit connected in shunt withthe input circuit of said main amplifier and an output circuit, means for combining said resultant voltage and the output from the output circuit. of saidv auxiliary amplifier and applying said combined. voltage .to the input circuits of said amplifiers, said auxiliary amplifier having asubstantially zero gain at the frequencies of said signal voltagesand. gain and phase shift characteristics outside of said band which alter the magnitude and. phase of feedback applied from the output of said. main amplifier to the input thereof, whereby the output of said main amplifier is applied to the input thereof in inverse phase relationship-over said band'of frequencies to improve the operation of said systemfor said band of frequencies and the feedback from the output to input of said main amplifier beyond said band is rendered insuflicient to cause selfoscillation of said main amplifier.

3. An amplifier system for amplifying voltages extending over a band of audio frequencies, a main amplifier having an input and an output circuit, a load circuit, a coupling network connecting said output circuit to said load circuit, said coupling network comprising a constant impedance coupling circuit for signal frequencies contained within said band and having an impedance which increases with increase in frequency outside of said band, means for deriving a voltage decreasing with increase in frequency beyond said band from said load circuit, means for deriving a second voltage increasing with increase in frequency beyond said band from said coupling network, means for adding said voltages whereby the phase of said resultant voltage remains substantially constant over a range of frequencies beyond said band, an auxiliary amplifier having an input circuit connected in shunt with the input circuit of said main am- 0 plifier and an output circuit, means for combining said resultant voltage and the output from the output circuit of said auxiliary amplifier and applying said combined voltage to the input circuit of said amplifiers, said auxiliary amplifier having a substantially zero gain at the frequencies of said signal voltages and having gain and phase characteristics outside of said band which alter the magnitude and phase of the feedback applied from the output of said main amplifier to the input thereof, whereby the output of said main amplifier is applied to the input thereof in inverse phase relationship over said band of frequencies to improve the stability of said main amplifier over a range substantially greater than that of said given frequency band.

4. An amplifier system for amplifying voltages extending over a band of frequencies, a main amplifier having an input and an output circuit, a load circuit, a coupling network connecting said output circuit to said load circuit, said coupling network being so proportioned to provide a constant resistive load substantially independent of frequency for signal frequencies contained within said band and an impedance varying with frequency outside of said band, means for deriving a voltage decreasing with increase in frequency beyond said band from said load circuit, means for deriving a second voltage increasing with increase in frequency beyond said band from said coupling network, means for adding said voltages whereby the phase of said resultant voltage remains substantially constant over a range of frequencies beyond said band, an auxiliary amplifier having an input circuit connected in shunt with the input circuit of said main amplifier and an output circuit, means for combining said resultant voltage and the output from the output circuit of said auxiliary amplifier and applying said combined voltage to the input circuits of said amplifiers, said auxiliary amplifier having a substantially zero gain at the frequencies of said signal voltages and gain and phase shift characteristics outside of said band which alter the magnitude and phase of feedback applied from the output of said main amplifier to the input thereof, whereby the output of said main amplifier is applied to the input thereof in inverse phase relationship over said band of frequencies to improve the operation of said system for said band of frequencies and the feedback from the output to input of said main amplifier beyond said band is rendered insufficient to cause self oscillation of said main amplifier.

CHARLES A. E. BEURTHERET.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,843,288 Leonard, Jr Feb: 2, 1932 2,162,744 Owendofi June 20, 1939 2,172,453 Rose Sept. 12, 1939 2,244,249 Guanella June 3, 1941 2,253,976 Guanella Aug. 26, 1941 2,480,163 Romander Aug. 30, 1949 FOREIGN PATENTS Number Country Date 550,171 Great Britain Dec. 28, 1942 

